There are two motions that take place when a vehicle turns. The first motion is a turning motion called yaw motion. Yaw motion takes place as the vehicle body spins around an imaginary vertical axis that is substantially perpendicular to the ground. The second motion is a lateral sliding motion called side-slip motion. Side-slip motion occurs in the same direction as the turn or in the opposite direction depending on the speed of the vehicle.
A desired yaw rate and a desired side-slip velocity are determined based on the speed of a vehicle and the position of the steering wheel. The desired values correspond to the expected yaw rate and side-slip velocity when a vehicle is traveling on a dry and clean surface. A driver may sense instability when either the actual yaw rate or side-slip velocity, or both of them, significantly exceeds the expected value.
In a vehicle stability enhancement system, the actual yaw rate and side-slip velocity of the vehicle are compared to their desired values. Corrective action is taken when either of their desired values is exceeded by a predetermined threshold. When a significant discrepancy exists between either the desired yaw rate and the actual yaw rate or the desired side-slip velocity and the actual side-slip velocity of the vehicle, or both of them, it is likely the road conditions necessitate vehicle stability enhancement.
Current methods of vehicle stability enhancement include using yaw rate feedback and side-slip acceleration feedback control signals. The yaw rate of a vehicle can be measured using a commercially available yaw rate sensor. The side-slip velocity of a vehicle can be measured using a side-slip velocity sensor, or sensors, which is currently very expensive. Instead of using a side-slip velocity sensor, side-slip acceleration can be estimated based on the lateral acceleration, yaw rate, and speed of a vehicle. Ideally, the side-slip velocity of a vehicle can be obtained by integrating the side-slip acceleration. However, since sensor bias exists in the signals associated with yaw rate sensors and lateral accelerometers, the integration tends to drift due to the integration of the unwanted bias signal.
In one conventional approach, a vehicle stability enhancement system uses yaw rate feedback and side-slip angle feedback (which can be derived from side-slip velocity) to create a corrective yaw moment to enhance stability and improve the dynamic behavior of a vehicle. The estimation of side-slip velocity is implemented using a dynamic observer that captures the estimated state of dynamics of the vehicle. However, the estimation is based on the cornering compliances of the vehicle corners, which are variable vehicle parameters. The cornering compliances vary over a wide range and depend on a number of factors, including the type of surface that the vehicle is operating on. Therefore, the estimate of side-slip velocity is not as accurate as desired.
In another approach, set forth in commonly-assigned, co-pending U.S. patent application Ser. No. 10/305,378 filed on Nov. 26, 2002, which is hereby incorporated herein by reference in its entirety, a vehicle stability enhancement control system uses an open loop rear wheel steering angle command in combination with rear wheel yaw rate feedback and rear wheel side-slip rate feedback to create a corrective yaw moment to enhance stability and improve the dynamic behavior of a vehicle.
In yet another approach set forth in commonly-assigned, co-pending U.S. patent application Ser. No. 10/404,371 filed on Apr. 1, 2003, which is hereby incorporated herein by reference in its entirety, a vehicle stability enhancement control system includes a side-slip velocity estimation module. A side-slip acceleration estimation module determines an estimated side-slip acceleration of a vehicle. In the side slip acceleration module, a limited-frequency integrator integrates the estimated side-slip acceleration to determine an estimated side-slip velocity of the vehicle. The estimated side-slip acceleration is determined based on a yaw rate, a lateral acceleration, and a speed of the vehicle. A reset logic module clears an output of the limited-frequency integrator when a first condition occurs. The first condition is a straight-driving condition that is determined based on a yaw rate, a lateral acceleration, and an angle of a steering wheel of the vehicle. The first condition is a speed condition that is based on a speed of the vehicle. The first condition is a sensor bias condition that is based on the estimated side-slip acceleration. The estimated side-slip velocity is compared to a desired side-slip velocity to produce a side-slip control signal. The side-slip control signal is combined with a yaw rate control signal to produce an actuator control signal. The actuator control signal is received by at least one brake actuator that applies a brake pressure difference across at least one axle of the vehicle to create a yaw moment to correct a dynamic behavior of the vehicle. The actuator control signal is received by a rear-wheel steering actuator that turns a set of rear wheels of the vehicle to create a yaw moment to enhance stability and improve the dynamic behavior of the vehicle. The limited-frequency integrator includes a feedback control loop. The accuracy of the integrator determines the accuracy of the side-slip velocity determined thereby, and hence, the accuracy and performance of the overall stability enhancement control system. The accuracy of the limited frequency integrator can be improved by improving the accuracy of the frequency cutoff associated therewith.
Therefore, it is desirable to improve the accuracy of the frequency cutoff of the limited frequency integrator in order to improve the accuracy and performance, and hence the usefulness, of the integrator, and in turn to improve the performance of the vehicle stability enhancement control system which is based thereon.